In recent years, a switching device for power conversion such as an IGBT (Insulated Gate Bipolar Transistor) has been widely applied, starting from low-power devices including a domestic air conditioner and a microwave oven to high power apparatuses for a railway and an iron factory. Since power conversion from a DC current to an AC current is indispensable to utilize renewable new energy and promote energy saving and vice versa, the switching device for power conversion has been an important key component for realizing a low-carbon society from now on.
Meanwhile, when the switching device for power conversion such as the IGBT is applied to an inverter for power conversion or the like, a conduction loss accompanying an on-resistance occurs during conduction, and a switching loss accompanying a switching operation occurs during switching. Therefore, the conduction loss and the switching loss are both required to be reduced in order to attain high efficiency and miniaturization of the inverter.
An example of an IGBT is disclosed in Patent Document 1, the IGBT being capable of enlarging a safe operation region at the turn-off without impairing the characteristics of low conduction loss, as a result of arranging a plurality of trench-type gates at equal intervals and by supplying control signals, having their turn-off timing shifted, to the mutually adjacent trench-type gates.
Moreover, a description is given in Patent Document 2 regarding an example of an IGBT that is capable of reducing a conduction loss, i.e., an on-state voltage without causing deteriorations in short-circuit tolerance and breakdown voltage in the following steps: alternately arranging a plurality of trench-type gates at two types of intervals different from each other; forming a channel layer (base region) and an emitter region above a semiconductor layer sandwiched by the two gates narrow in the gate interval; and forming a floating layer, which is not connected to an emitter electrode, on a semiconductor layer sandwiched by the two gates wide in the gate interval.
According to the examinations by the present inventors of the present application, however, it has been found that the IGBT having the structure disclosed in Patent Document 2 has problems in that a turn-off loss is large and controllability of the rate of temporal change in the output voltage dv/dt of each diode of the IGBT or a pair of arms deteriorates at the time of turn-on.
One of these problems, which is regarding the deterioration of the controllability of the rate of temporal change in the output voltage (dv/dt) at the turn-on, has been described in Patent Document 3 along with the reason of its occurrence as follows.
Since holes transiently flow into a p-type floating layer formed between two gates when the IGBT is brought to an on-state, the potential of the floating layer turns out to be high. At this time, a displacement current flows in each gate through a feedback capacity of a gate insulating film that separates the gate from the floating layer, thereby raising the potential of the gate. As a result, the rate of temporal change in collector current (ic) (dic/dt) determined by the product of the mutual conductance (gm) of a MOS (Metal Oxide Semiconductor) FET (Field Effect Transistor) structure and the rate of temporal change in gate-emitter voltage (vge) (dvge/dt) increases, and a switching speed is accelerated.
Since the amount of the holes that transiently flow into the floating layer is principally determined by a semiconductor internal structure, it is hard to control it with an external gate resistor. Accordingly, there is generated a period during which the accelerated dic/dt cannot be controlled with the external gate resistor, such that the rate of temporal change in the voltage dv/dt of each of the diodes for the IGBT and the pair of arms cannot be controlled with the gate resistor.
With these taken into consideration, there has been proposed in Patent Document 2 an IGBT having a structure that makes a parasitic capacitance less likely to be generated. The method of that includes thickening an insulating film between a drift layer or floating layer and a gate electrode. The feedback capacity will be smaller too as long as the parasitic capacitance between the above layer and the gate electrode is small. The controllability of the rate of temporal change in the output voltage (dv/dt) at the time of turn-on is improved.